John 1:17 Verse 17 mentions "law and grace" which is the same as "law and gospel." Both are necessary. Moses and the Old Testament followers had the law (cold stone tablets), but grace and truth are in the New Testament through Jesus Christ (warm living life). The law is still here, but Christ fulfills the law. Matthew 5:17, "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them."
John 1:18 "No one has seen God, the only God, who is at the Father's side, he has made him known." This would describe Jesus who is the "only begotten son." John references before Jesus was on earth (Moses as in the previous verse) and the God in heaven who is not seen or known. After the "comma," John references after Jesus is born when the people have seen the earthly "man" their own eyes when He was made known. However, they would not "really" see Him as in having knowledge of the truth.